Pyrolytic coatings of molybdenum sulfide by plasma jet technique

ABSTRACT

A plasma-forming fluid consisting of a monatomic gas and at least one other gas having a diatomic molecule is employed to pyrolytically produce chemical elements and chemical compounds, such, for example, as pyrolytic carbon and refractory carbides. Pyrolytic carbon having a density of from 2.1 to 2.25 grams per cubic centimeter and an impurity content of less than 20 parts per million is made from a reactant material comprising aliphatic halides of carbon introduced into the plasma-forming fluid.

United States Patent Chu [ 51 Apr. 25, 1972 154] PYROLYTIC COATINGS OFMOLYBDENUM SULFIDE BY PLASMA JET TECHNIQUE [72] Inventor: Ting Li Chu,Dallas, Tex.

[73] Assignee: Westinghouse Electric Corporation, Pittsburgh, Pa.

doned.

[52] U.S.Cl ..117/93.l PF,117/105.2, 117/121, 117/127, 252/31, 23/206[51] Int. Cl ..B05b 7/22 [58] Field ofSearch ..117/93.1PF, 46 ES,I05,105.1, 117/1052; 23/206, 208, 209.1, 209.4, 209.3; 204/174; 252/25,31

[56] References Cited UNITED STATES PATENTS 2,922,869 1/1960 Giannini etal ..l17/93.1 PF 2,960,594 11/1960 Thorpe ....117/93.1 PF 3,246,1144/1966 Matvay ....l17/93.1 PF 3,275,408 9/1966 Winterburn ..117/105.2

3,331,664 7/1967 Jordan .23/209.3 3,346,338 10/1967 Latham....

3,411,949 11/1968 Hough ..117/93.l PF 3,458,341 7/1969 Diefendorf..23/209.4 3,485,591 12/1969 Evans et a1. ..23/208 A 3,503,787 3/1970Pendse ..ll7/93.l PF

FOREIGN PATENTS OR APPLICATIONS OTHER PUBLICATIONS Davis Metal ProgressVol. 83 No. 3 March 1963 pp. 105- 108,142,144,146,148

Primary Examiner-Alfred L. Leavitt Assistant Examiner.l. I-I. NewsomeAttorney-F. Shapoe and C. L. Menzemer [57] ABSTRACT A plasma-formingfluid consisting of a monatomic gas and at least one other gas having adiatomic molecule is employed to pyrolytically produce chemical elementsand chemical compounds, such, for example, as pyrolytic carbon andrefractory carbides. Pyrolytic carbon having a density of from 2.1 to2.25 grams per cubic centimeter and an impurity content of less than 20parts per million is made from a reactant material comprising aliphatichalides of carbon introduced into the plasma-forming fluid.

1 Claim, 3 Drawing Figures Great Britain ..1 17/93.! PF

PYROLYTIC COATINGS OF MOLYBDENUM SULFIDE BY PLASMA JET TECHNIQUECROSS-REFERENCES TO RELATED APPLICATION ry materials is well known. Theplasma jet plating techniquehas also been used for the fabrication ofsemiconductor devices to provide ohmic contacts and junctions.

In the conventional jet plating technique, the chemical element orcompound, usually in the powder form, is introduced into a plasma jetwhere it receives energy by collisions with the plasma particles. Theplasma jet then transports the element or compound involved to thesubstrate surface at high directed velocities and it is deposited as acoherent-andadherent coating upon the suitable substrate. The densityand uniformity of the coating depends upon many factors suchas theenthalpy of the plasma jet and the physical properties of the materialbeing deposited, such as melting point, particle size, the rate anduniformity of its introduction into the plasma jet stream and the like.Unless the material being deposited-is melted, or finely dispersed, inthe plasma jet, the deposited material usually has a density lower thanits true density and the coating does not have optimum uniformity andstructure.

This situation prevails when a highly refractory material, such astungsten, or carbon is used.

An object of this invention is to provide a method for depositing animproved, uniformly coherent and adherent, dense coating of chemicalelements and chemical compounds of controlled thickness upon anysuitable substrate surface employing a plasma forming fluid consistingof a mixture of monatomic and diatomic gases.

Another object of this invention is to provide a method .for depositingan improved, uniformly coherent and adherent,

dense coating of pyrolytic carbon and refractory carbides.

upon suitable substrate surfaces for high temperature applications onany size or shape of substrate surface by employing a plasma formingfluid consisting of a mixture of a monatomic and a diatomic gas.

Other objects of this invention will, in part, be obvious and will, inpart, appear hereinafter.

In order to more fully understand the nature and objects of thisinvention, reference should be had to the following detailed descriptionand drawings in which:

FIG. 1 is a cross-sectional view ofthe plasma jet generator employed inproducing pyrolytic coatings by a plasma jet technique embodying theteachings of this invention;

FIG. 2 is a cross-sectional view of an improved pyrolytic coating ofmaterial on a surface of a substrate; and

FIG. 3 is a view, partly in cross-section, showing themethod ofdepositing an improved pyrolytic coating of a material employing anotherplasma jet technique embodying the teachings of this invention.

In accordance with the teachings of this invention and in the attainmentof the foregoing objects, there is provided an improved coating ofdense, uniformly coherent and adherentpyrolytic carbon such, forexample, as carbon in theformof graphite, on a suitable substrate. Thepyrolytic coating of carbon has a bulk density of from 2.1 to 2.25 gramsper cubic centimeter and an impurity content of less than 20.parts permillion.

A preferred method for depositing the improved-coating ofcarbon is tointroduce one or more suitable chemical elements and chemical compounds,in the form of a gas or a low boiling point fluid, into an ionizedplasma-forming fluid or plasma jet.

This preferred method permits a more uniform introduction of a reactantmaterial at a predetermined, controlled rate into a plasma jet.

Excellent results are obtained when a pyrolytic carbon coating is formedon a carbon substrate by utilizing aliphatic and aromatic halides ofcarbon. Examples of suitable aliphatic halides are tetrachloromethane,tetrabromomethane, trichloromethane, tribromomethane, dichloromethane,dibromomethane, monochloromethane, monobromomethane, monochloroethane,monobromoethane, monochloropropane, monobromopropane and the like.Suitable aromatic halides are monochlorobenzene, monobromobenzene,dichlorobenzene, dibromobenzene, chlorotoluene,

bromotoluene and the like.

A plasma-forming fluidconsisting of a monatomic gas and at least oneother gas having a diatomic molecule has proven to be very satisfactoryinproducing the improved pyrolytic coating ofcarbon; A suitableplasma-forming fluid comprises argon gas as the monatomic gas andhydrogen gas in quantities up to 10 mole percent as the diatomic gas.

Hydrogen is chosen as the diatomic gas ingredient because of its highinternal energy and its high affinity for halides. The amount-ofhydrogen present in the plasma jet is always many times greater thanthat required for the reaction in the carbon carrying reactant materialintroduced into the ionized plasmaforming fluid. Depending on thereactant material, a chemical reaction occurs between the hydrogen gasand the reactant material, or chemical dissociation of the reactantmaterial occurs in the plasma-forming fluid, producing carbon which isthen deposited as the improved pyrolytic coating. Other reactantproducts are carried off as a gas.

The amount of contaminants in the improved pyrolytic carbon coatingresulting from a process embodying the teachings of this invention isonly 20 parts per million maximum.

With reference to FIG. 1, there is shown a plasma jet generator 10suitable for depositing the improved pyrolytic coating of carbon on asuitable substrate material.

The plasma-jet generator is comprised of an arc chamber 12 and a productchamber 14.

The are chamber l2is defined by a circular metal wall 16 having anoutwardly extending integral metal peripheral flange 18.at one end andis enclosed by an anode 20'disposed at one endof the wall 16 and a backplate 22 disposed at the other end of the wall 16. The wall 16 is madeof brass and contains an integral coolant passage 24 for the circulationof a coolant such, for example, as water.

A plasma fluid inlet tube 26 passes through the wall 16 and into the arcchamber 12. The plasma fluid inlet tube 26 permits the plasma-formingfluid to be introduced directly into the enclosed arc chamber 12.

The wall 16 is. joined to the anode 20 by disposing the peripheralflange 18into a recessed portion 28.0f an integral peripheral flange 300f the anode 20. The flanges l8and 30 may. be secured by any suitablemechanical means.

A gas tight seal 32between the wall 16 and the anode 20 is obtained byemploying an O-ring type seal 32. The seal 32 is disposed in an annulargroove 34=of the flange 30.

The anode-20 contains an integral passage 36 for the circulation of acoolant'such, for example, as water and has an axially disposedfunnelshaped bore 38. The bore 38 has an entrance.40and an exit 42. Thediameter ofthe entrance 40 is greater than the diameter of the exit 42.

A reactant inlet tube 44- passes completely through the anode 20 and.enters into the bore 38. The reactant inlet tube 44, isdisposedperpendicular to the axis of the bore 38. The reactant inlet tube 44permits reactant material to be introduced into the plasma jet streambeyond the arc column.

An insert 46. isdisposed within the bore 38. In producing pyrolyticcarbidesand pyrolytic carbon the insert 46 is made of carbon in-the formof graphite. The insert 46 prevents the erosionaof-the anode-20'duringthe operation of the generator 10, and thereby prevents any'brass fromcontaminating the yrolytic coating of material which is formed. Anoutwardly extending flange 48 is seated within a recess 50 of the bore38 to permit a flush contour surface within the bore 38. A hole 52permits the insertion of the reactant inlet tube 44 through the insert46 and into the bore 38.

The back plate 22 is made of brass and is joined to the wall 16 bysuitable mechanical means. A gas tight seal is achieved by disposing anannular gasket 54 of an electrically insulating material between theback plate 22 and the wall 16. The back plate 22 has an axially disposedraised embossment 56.

A cathode holder 58 is disposed through an aperture 60 in the embossment56. A gas tight seal is achieved between the holder 58 and the backplate 22 by an O-ring type gasket 62 disposed in an annular groove 64within the aperture wall 66.

The cathode holder 58 is made of silver plated brass. The holder 58 hasan integral passageway 68 to permit the circulation of a coolant, such,for example, as water. The holder 58 also has a recessed means 70 forsupporting a cathode 72 inserted therein.

The cathode 72 is made of a material selected from the group consistingof carbon, tungsten, tantalum, molybdenum and alloys thereof.

Since ablation of the cathode 72 is a natural occurrence during theoperation of the generator 10, carbon in the form of graphite is apreferred material for making the cathode 72 used in pyrolyticallydepositing coatings of carbon and carbides.

The product chamber 14 is made of silver plated brass and is joined tothe anode 20 by suitable means. The product chamber 14 is where thepyrolytic coatings of this invention are deposited on suitablesubstrates. A viewing port 74 in a wall 76 of the chamber 14 permits oneto have easy access for measuring the temperature of a substrate surfaceas well as allowing a means for locating a substrate properly in respectto the stream of the plasma flame.

The plasma-forming fluid consisting of argon gas with up to 10 molepercent of hydrogen gas is introduced into the enclosed arc chamber 12through the plasma fluid inlet tube 26. A preferred range for theplasma-forming fluid comprises argon gas containing from 0.8 molepercent of hydrogen gas to 3.8 mole percent of hydrogen gas. Theplasma-forming fluid has a gas flow rate of from 0.8 gram-moles perminute to 3.0 gram-moles per minute. A preferred flow rate is 1.25gramsmoles per minute.

The plasma jet generator arc current range is between 200 amperes and1,000 amperes.

At the preferred gas flow rate of 1.25 gram-moles per minute, aplasma-forming fluid comprised of argon gas containing 3.8 mole percentof hydrogen gas, when ionized by an arc current of 300 amperes, has aspecific enthalpy of 56 Kilocalories per gram-mole of the plasma-formingfluid. This is equivalent to a temperature of approximately 9,400" K.

A substrate 78, such, for example, as carbon in the form of graphite, isdisposed within the product chamber 14 at a distance of from 2 tocentimeters from the exit 42 of the axial bore 38 of the anode 20. Apreferred distance for disposing the substrate 78 is to place it 3centimeters in front of the exit 42 of the axial bore 38.

The ionized plasma-forming fluid heats a surface 80 of the substrate 78to an elevated temperature. It is upon this heated surface 80 that apyrolytic coating, in this instance carbon, is deposited.

Since normal operation of the plasma jet generator produces an ionizedplasma-forming fluid, or plasma jet, having a temperature ofapproximately 9,400" K. at the exit 42 of the bore 38, the temperatureof the surface 80 of the substrate 78 heated by the plasma jet will varyfrom approximately l,200 to 3,000 K. At the optimum positioning of thesubstrate 78 in the product chamber 14, the average temperature of thesurface 80 is approximately 2,500 K.

The substrate 78 is heated directly by the plasma jet, and not from anexternal source. Any size or shape of substrate 78 may becoated bymethods embodying the teachings of this invention. The "substrate 78 maybe moved in any direction within a plane perpendicular to the centerlineaxis of the plasma jet discharge and so obtain a uniform coating over alarge area of the surface 80 of the substrate 78.

Conversely, the substrate 78 may be held stationary and the plasma jetprogrammed to direct its discharge over the changing surface 80.irregularly shaped, and sizes of, substrates may also be programmedaccordingly in order to achieve a uniform coating over the entiresurface 80 of the substrate 78.

In choosing the proper reactants for the desired end product to beproduced, one has to know the enthalpy change of the chemical reactionor dissociation which will occur in the plasma jet, the enthalpy of theplasma-forming fluid and the internal energy to be expected within theplasma jet. It is most desirable that the required reactants areintroduced at'that given rate which is sufficient to allow the internalenergy of the plasma jet to supply the necessary heat of reaction ordissociation to carry all expected chemical reactions or dissociationsto completion. Therefore, depending upon the heat of reaction ordissociation of the expected chemical reactions or dissociations andemploying the teachings of this invention, reactants can be introducedinto the plasma jet at rates up to 10 gram-moles per hour.

With added reference to FIG. 2 there is shown a pyrolytically coatedsubstrate 100..The pyrolytically coated substrate consists of a carbonsubstrate 102 and a layer 104 of dense, coherent and adherent pyrolyticcarbon disposed on a surface 106.

One or more of the aliphatic and aromatic halides of carbon isintroduced into the plasma jet generator 10 through the reactant inlettube 44 at a rate of from 0.01 gram-mole per minute to 0.2 gram-mole perminute. The optimum flow rate is 0.1 gram-mole per minute.

The layer 104 of improved dense, uniformly coherent and adherentpyrolytic carbon is deposited upon the heated surface 106 of the carbonsubstrate 102 at a rate of between 0.05 grams per minute to 1.0 gramsper minute. An average deposition rate of 0.1 grams per minute isequivalent to a thickness of 0.005 inch of pyrolytic carbon beingdeposited on 2 square centimeters of substrate 102 per minute. Comparedto previously known conventional methods, this is a substantial increasein the plating or coating of a substrate with a pyrolytic carbon.

The layer 104, or pyrolytic coating, of carbon deposited is extremelydense when compared to commercial graphite known previously to thoseskilled in the art and has a density of from 2.1 to 2.25 grams per cubiccentimeter. Being a more uniform coating, the improved pyrolytic coating104 of carbon afiords great reliability for high temperatureapplications in the aeronautical industries and similar endeavors. Aparticular application of this improved pyrolytic coating 104 of carbonis in the design of rocket propulsion system nozzles and in the throatsections of the venturi portion of a rocket engrne.

Refractory carbides are also deposited upon suitable substrates, such ascarbon, tungsten, molybdenum and the like, utilizing the same process asheretofore disclosed for the production of pyrolytic carbon. A mixtureof volatile compounds of the metal and carbon is used as the reactantfluid for the preparation of the refractory carbides by the plasma jettechnique. This process also employs a mixture of monatomic and diatomicgases such, for example, as helium and hydrogen, helium and nitrogen,argon and hydrogen and argon and nitrogen, as a plasma-forming fluid.

The reactants decompose in the plasma jet and the dis sociated elementsreact with one another to form the required carbide which is transportedby the plasma jet fluid to be deposited upon a substrate surface in adense, uniformly coherent and adherent pyrolytic coating. The halide ofthe reactant gas is exhausted from the plasma jet generator.

Suitable reactant metal compounds for forming refractory carbides by theembodiments of the teachings of this invention are the compounds ofmetal and metallic halides. These metallie halides include halides ofsilicon such for example, as trichlorosilane, silicon tetrachloride,silicon tetrabromide, chlorides of boron, aluminum, titanium, zirconium,molybdenum and tungsten, as well as the bromides of boron, aluminum,titanium, zirconium, molybdenum and tungsten. Suitable carbon reactantsare the same as those used in the depositing of pyrolytic carbonembodying the teachings of this invention.

Employing solid chemical compounds with low sublimation anddecomposition temperatures, it has been found that metal carbonylsselected from a group of metals consisting of tungsten, chromium,molybdenum, iron, cobalt, nickel, rhenium, iridium and osmium are themost suitable for depositing pyrolytic coatings of metal carbides uponsuitable substrates utilizing the plasma jet technique. The metalliccarbonyls decompose into a metal and carbon monoxide at relatively lowtemperatures such, for example, as tungsten, carbonyl,

molybdenum, carbonyl and chromium carbonyl, all of which decomposewithout melting at approximately 150 C.

The metallic carbonyls are reacted with aliphatic and aromatic carbonhalides in a plasma-forming fluid comprising a mixture of monatomic anddiatomic gases, such, for example as a mixture of argon and hydrogencontaining up to mole percent of hydrogen, resulting in a uniformlydense, coherent and adherent pyrolytic coating of a metallic carbideupon the surface of a suitable substrate. The same aliphatic andaromatic carbon halides hereinbefore described as reactants withmetallic halides for the production of metallic carbides embodying theteachings of this invention are suitable reactant materials.

Other suitable reactant materials for obtaining metallic carbidecoatings upon suitable substrate surfaces are found in the group ofmetallo-organic compounds. These metallo-organic compounds yield a metalcarbide upon thermal decomposition without the plasma jet. The metalcarbide coating is produced by the same general plasma jet methodshereinbefore described by the teachings of this invention.

It has been found that each of the metallo-organic compounds ofmethyltrichlorosilane and aluminum triethyl yield respective pyrolyticcoatings of silicon carbide and aluminum carbide. Other suitablemetallo-organic compounds which yield aluminum carbide aretrialkylaluminum, tri-n-propylaluminum, tri-n-butylaluminum,tri-i-propylaluminum and di-isobutyl-aluminum hydride.

Other chemical elements and chemical compounds are deposited on suitablesubstrate surfaces using the plasma jet technique embodying theteachings of this invention. When molybdenum carbonyl and hydrogensulfide are used as the reactants in a plasma-forming fluid comprisingargon and hydrogen, the resulting deposit of molybdenum sulfidedeposited on a graphite substrate was found to be both coherent andadherent as well as uniformly dense throughout its structure.

Employing the embodiment of the teachings of this invention, uniformlydense, coherent and adherent coatings of chemical compounds such asoxides, carbides, borides and silicides are also feasible.

With reference to FIG. 3 there is shown another means of depositing adense uniformly coherent and adherent coating 150 of chemical elementsand chemical compounds on a surface 152 of a suitable substrate 154.This means utilizes the plasma jet generator 10 of FIG. 1 as a means toheat the surface 152 with an ionized plasma-forming fluid 156 andimpinge reactant material 158 upon the heated surface 152.

While the ionized plasma-forming fluid 156 continues to heat the surface152 of the substrate 154, the reactant material 158, in a gaseous form,is caused to impinge upon the heated surface 152. In one instance thereactant material 158 chemically reacts with the ionized plasma-formingfluid 156 to produce the required material for the coating 150. Anyother chemical products resulting from the chemical reaction passes offas a gas.

A second possible reaction which the reactant material 158 may undergois to chemically decompose upon impinging with the heated surface 152instead of chemically reacting with the ionized plasma-forming fluid156. Upon chemically decomposing, the required material for the coatingis deposited on the surface 152 and the remaining portion of thedecomposed reactant material 158 passes off as a gas.

The reactant material 158, in a gaseous form, flows through a reactanttube which controls its direction of flow. The direction of flow of thegaseous reactant material 158 is such that the theoretical centerlinesof travel of the reactant material 158 and the ionized plasma-formingfluid 156 will intersect on the surface 152 of the substrate 154 wherethe layer 150 of pyrolytic material will be deposited.

The reactant inlet tube 160 is made of a suitable material, such, forexample, as glass, metal and the like. The material is one that is inertto the reactant material 158 which is flowing through the tube 160.Although the tube 160 must be disposed close enough to the surface 152to assure proper impingement of the reactant material 158 thereon, theheat is not sufficient to have any deleterious effects on the tube 160.

The reactant material 158 flows at a sufficient velocity to overcome theeffects occasioned by the velocity of the ionized plasma-forming fluid156 impinging on the surface 152 of the substrate 154.

With both the ionized plasma-forming fluid 156 and the reactant material158 impinging the surface 154 in the same area, good mixing of them willoccur. Therefore, should the reactant material 158 be a material thatwill chemically decompose to form the layer 150, a good thermal transferenergy is obtained to accomplish the required chemical decomposition. Ifthe reactant material 158 is a material that will chemically react witha portion of the ionized plasmaforming fluid 156, the mixing of thereacting components is easily accomplished.

In either instance, the resulting required material desired for thepyrolytic layer 150 is deposited on the surface 152. Any other chemicalproducts resulting from either the chemical decomposition, or thechemical reaction, of the reactant material 158 is exhausted by suitablemeans from the surface 152 as a gas as the velocity of the reactantmaterial 158, is sufficient to aid the flow of gaseous products awayfrom the sur face 152 as well as being able to overcome the effects ofthe velocity of the ionized plasma-forming fluid 156.

The following examples are illustrative of embodiments of the teachingsof this invention.

EXAMPLE I Embodying the teachings of this invention pyrolytic carbon wasdeposited by the plasma jet technique upon a carbon substrate usingchloroform as a reactant material. The arc current for the plasma jetwas 300 amperes. The specific enthalpy of the plasma jet was 56kcal/gram-mole or a temperature approximately 9,400 K. The graphitesubstrate was disposed 3 centimeters from the plasma jet nozzle and itssurface temperature was approximately 2,500 K. as a result of heatingfrom the plasma jet. The reactant fluid of chloroform was introducedinto the plasma jet through an inlet tube in the nozzle at a rate of 0.1gram-mole per minute. The anode nozzle and the cathode of the plasma jetgenerator were made of high purity carbon in the form of graphite.

The plasma-forming fluid consisted of argon gas containing 3.8 molepercent of hydrogen gas. The plasma-forming fluid was introduced intothe plasma jet generator at a gas flow rate of 1.25 moles per minute.

The resulting pyrolytic carbon was deposited upon the carbon substrateat an average rate of 0.1 gram per minute and had a bulk density of 2.1to 2.25 grams per cc. The resulting pyrolytic carbon had the samestructure as pyrolytic carbon produced by previously known methods ofthe art as determined by X-ray diffraction patterns. The impuritycontent of the pyrolytic carbon was less than 20 ppm as determined byspectroscopic analysis. Microscopic examination of the interface betweenthe substrate surface and the pyrolytic carbon showed a very coherentand adherent structure.

EXAMPLE II Embodying the teachings of this invention silicon carbide wasdeposited by the plasma jet technique upon a surface of a graphitesubstrate using a mixture of trichlorosilane and propylene as thereactants. The are current employed in the plasma jet generator was 300amperes. The specific enthalpy of the plasma jet was 56 kcal/gram-moleor a temperature approximately 9,400 K. A graphite substrate wasdisposed 4 centimeters from the plasma jet nozzle and the substratesurface temperature was approximately l,600 C. as a result of heatingfrom the plasma jet. The mixture of reactants was introduced into theplasma jet at a rate of 0.4 gram-moles per hour for propylene and 2.0grams-moles per hour for the trichlorosilane.

The plasma-forming fluid consisted of argon gas containing 3.8 molepercent of hydrogen gas. The plasma-forming fluid was introduced intothe plasma jet generator at a gas flow rate of 1.25 moles per minute.

A deposit of dense uniformly coherent and adherent silicon carbide wasdeposited on the graphite substrate at a rate of 0.15 grams per minute.Upon examination, the structure of the silicon carbide was found to becubic.

EXAMPLE Ill Embodying the teachings of the invention a coating ofmolybdenum sulfide was deposited by the plasma jet technique upon agraphite substrate, disposed 5 centimeters from the plasma jet nozzle,using a reactant mixture of molybdenum carbonyl and hydrogen sulfidegas. The are current for the plasma jet generator was 300 amperes. Theplasma-forming fluid consisted of argon gas containing 3.8 mole percentof hydrogen gas. The plasma-forming fluid was introduced into the plasmajet generator at a gas flow rate of 1.25 moles per minute. The specificenthalpy of the plasma jet was 52 kcal/gram-mole or a temperature ofapproximately 9,300 K. The graphite substrate had a surface temperaturethat was approximately l,300 K. as a result of heating from the plasmajet. The reactant mixture of molybdenum carbonyl and hydrogen sulfidewas introduced into the plasma jet through an inlet tube in the nozzleat a rate of 0.5 gram-moles per hour for molybdenum carbonyl and 2gram-moles per hour for the hydrogen sulfide. The anode nozzle and thecathode of the plasma jet generator were made of copper and tungsten,respectively.

The resulting deposit of molybdenum sulfide was deposited upon thesurface of the graphite substrate at a rate of 0.3 grams per minute.

Suitable substrate materials which can be used in conjunction with theteachings of this invention are dependent upon the end use of theresulting product. Carbon, in the form of graphite, is the mostconvenient substrate to employ since the coated substrate has a moreuniversal end use. However, coatings of refractory carbides can also bedeposited on the respective metals employed in the refractory carbides,such as tungsten for tungstencarbide, molybdenum for molybdenum carbideand the like. Other suitable substrates are iron, cobalt, nickel,vrhenium, osmium, chromium, boron, aluminum, zirconium and titanium.

In employing the impingement technique for depositing the coating ofchemical elements and chemical compounds, the same reactants disclosedin the preferred method, as well as the same type of plasma-formingfluid, the same plasma generator and the same operating conditions isutilized. Again, suitable substrates to be employed will be dependentupon the end use of the resulting product.

This method is similar to the pyrolytic coating process in which thesubstrate is heated by resistance or inductive heating. The technique ofheating a surface of a substrate with an ionized plasma-forming fluid ismore flexible however.

While the invention has been descnbed with reference to particularembodiments and examples, it will be understood, of course, thatmodifications, substitutions and the like may be made therein withoutdeparting from its scope.

I claim as my invention:

1. A process for producing a pyrolytic coating on a surface of asubstrate comprising;

1. introducing a plasma-forming fluid into a plasma jet generator at agas flow rate of from 0.8 gram-moles per minute to 3.0 gram-moles perminute, said plasma-forming fluid consisting of a mixture of argon andfrom 0.8 to 10 mole percent of hydrogen,

2. ionizing said plasma-forming fluid with an electrical current of from200 to 1,000 amperes,

3. introducing a reactant material consisting of a mixture of molybdenumcarbonyl and hydrogen sulfide into said ionized plasma forming fluidwhereby said reactant material undergoes a chemical change within saidionized plasma-forming fluid to form a reaction product of molybdenumsulfide which is transported by said ionized plasma-forming fluid to asubstrate where at least a portion of said reactant product is depositedon a heated surface of said substrate in a dense, uniform, coherent andadherent coating.

2. ionizing said plasma-forming fluid with an electrical current of from200 to 1,000 amperes,
 3. introducing a reactant material consisting of amixture of molybdenum carbonyl and hydrogen sulfide into said ionizedplasma forming fluid whereby said reactant material undergoes a chemicalchange within said ionized plasma-forming fluid to form a reactionproduct of molybdenum sulfide which is transported by said ionizedplasma-forming fluid to a substrate where at least a portion of saidreactant product is deposited on a heated surface of said substrate in adense, uniform, coherent and adherent coating.